Apparatus for controlling current through a plurality of resistive loads

Abstract

Disclosed is an electrical control circuit (10) for controlling current through a plurality of resistive loads (14(1), 14(2) . . . 14(N)). The control circuit includes a plurality of current control stages (12(1), 12(2), . . . 12(N)), a comparator circuit (18), and a plurality of diodes D 1 , D 2 . . . D N-1 in serial connection with the current control stages. The comparator circuit controls the current that is drawn collectively through the loads. The diodes ensure that all preceding current control stages in the series are conducting a predetermined level of current before the next current control stage begins to source current through its respective load. Each current control stage includes an operational amplifier (30), a pass transistor (34), a sense resistor (R s ), and a feedback resistor (R f ). The operational amplifiers are a pre-drive to the pass transistors to allow smaller sense resistors to be used, thereby providing greater efficiency. The feedback resistors (R f ) provide feedback to the operational amplifiers.

The embodiments of the invention in which an exclusive property or privilege is claimed one defined as follows: 
     
       1. Apparatus for controlling current through a plurality of loads using a single control line, comprising: a plurality of current control stages arranged in a sequence, each current control stage including a first terminal coupled to a different one of the loads, a second terminal, a third terminal, and means for conducting current between the first and second terminals in response to a level signal present at the third terminal, each current control stage being operable in response to its level signal in a first state in which it conducts a relatively low level of current, in a second state in which it conducts a predetermined level of current, and in a plurality of intermediate states between the first and second states in which the current control stage conducts current between said low and predetermined levels of current; and   activation means, coupled to the third terminals of each current control stage, for producing the level signals in response to a control signal present on the single control line, the activation means producing level signals such that when one of the current control stages is conducting current in an intermediate state, all preceding current control stages in the sequence are in their second states and all subsequent current control stages in the sequence are in their first states.   
     
     
       2. The apparatus of claim 1 wherein the current control stages are substantially non-conducting when in their first states. 
     
     
       3. The apparatus of claim 2 wherein the current control stages are fully conducting when in their second states. 
     
     
       4. The apparatus of claim 3 wherein the intermediate states of each current control stage form a continuum in which the amount of current conducted varies monotonically with its respective level signal. 
     
     
       5. The apparatus of claim 4 wherein the amount of current conducted varies in a substantially linear manner as the level signal is varied. 
     
     
       6. The apparatus of claim 1 wherein the activation means includes a plurality of blocking circuits, a different one of the blocking circuits being positioned between the third terminals of pairs of the current control stages, the blocking circuits having a substantially nonconducting state and a conducting state, each blocking circuit being in a conducting state only when all preceding current control stages are conducting current at their respective second states; and   a comparator circuit, responsive to the control signal and a feedback signal, for generating an output signal that is coupled to the third terminal of the first current control stage in the sequence, the control signal being a function of the desired level of current conducted by the current control stages, the feedback signal being indicative of the actual level of current being conducted by the current control stages and the output signal being a function of the voltage differential between the control and feedback signals.   
     
     
       7. The apparatus of claim 6 and further including a plurality of resistors, a different one of the resistors being coupled between the comparator circuit and the second terminals of each current control stage to thereby provide the feedback signal to the comparator circuit. 
     
     
       8. The apparatus of claim 6 wherein each current control stage includes a current probe positioned so as to sense the level of current being conducted by the current control stage. 
     
     
       9. The apparatus of claim 2 wherein each current control stage includes an operational amplifier, a pass transistor, a current sense resistor and a feedback resistor, the operational amplifier having an inverting input terminal, a noninverting input terminal coupled to receive the respective current control stage's level signal, and an output terminal, the pass transistor having a control terminal, an input terminal corresponding to the first terminal of the current control stage and an output terminal, the current sense resistor being connected between the output terminal of the pass transistor and a reference potential, the feedback resistor being connected between the inverting input of the operational amplifier and the output terminal of the pass transistor, each current control stage further including means for coupling the output of the operational amplifier to the control terminal of the pass transistor. 
     
     
       10. The apparatus of claim 9 wherein the pass transistor is a field-effect transistor. 
     
     
       11. The apparatus of claim 9 wherein the means for coupling comprises a drive transistor, the drive transistor having a control terminal, an input terminal and an output terminal, the control terminal of the drive transistor being connected to the output terminal of the operational amplifier and the output terminal of the drive transistor being coupled to the control terminal of the pass transistor to drive the pass transistor in response to an output signal generated by the operational amplifier. 
     
     
       12. The apparatus of claim 11 wherein the drive and pass transistors are bipolar transistors. 
     
     
       13. The apparatus of claim 9 wherein each blocking circuit comprises a diode. 
     
     
       14. The apparatus of claim 1 and further including a minimum circuit coupled between the single control line and the activation means, the minimum circuit establishing a maximum level of current to be conducted through the current control stages. 
     
     
       15. The apparatus of claim 14 wherein the minimum circuit is responsive to the control signal on the single control line and a limit signal, the apparatus further including a limit circuit for generating the limit signal such that the limit signal is representative of a maximum level of current to be conducted through the current control stages. 
     
     
       16. The apparatus of claim 15 wherein the minimum circuit includes (a) first and second operational amplifier responsive to the control and limit signals, respectively, each operational amplifier having an inverting input terminal, a noninverting input terminal, and an output terminal, (b) means, responsive to the greater of the control and limit signals, for blocking the output of the first and second operational amplifiers, and (c) first and second feedback resistors, coupled between the means for blocking and the inverting inputs of the first and second operational amplifiers, respectively. 
     
     
       17. The apparatus of claim 16 wherein the means for blocking includes first and second diodes, each having an anode and a cathode, the cathodes of the first and second diodes connected to the output terminals of the first and second operational amplifiers, respectively, and the anodes of each diode coupled to each other and to the first and second feedback resistors. 
     
     
       18. The apparatus of claim 1 and further including a plurality of minimum circuits, one of the minimum circuits being coupled between the activation means and each of the current control stages, the minimum circuit establishing a maximum level of current to be conducted individually by each current control stage. 
     
     
       19. The apparatus of claim 18 wherein each minimum circuit is responsive to the level signal present at each respective current control stage and a limit signal, the apparatus further including a limit circuit for generating the limit signal such that the limit signal is representative of the maximum level of current to be conducted by each current control stage. 
     
     
       20. The apparatus of claim 19 wherein each minimum circuit includes (a) first and second operational amplifiers responsive to the level and limit signals, respectively, each operational amplifiers responsive to the level and limit signals, respectively, each operational amplifier having an inverting input terminal, a noninverting input terminal, and an output terminal, (b) means, responsive to the greater of the level and limit signals, for blocking the output of the first and second operational amplifiers, and (c) first and second feedback resistors, coupled between the means for blocking and the inverting inputs of the first and second operational amplifiers, respectively. 
     
     
       21. The apparatus of claim 20 wherein the means for blocking includes first and second diodes, each having an anode and a cathode, the cathodes of the first and second diodes connected to the output terminals of the first and second operational amplifiers, respectively, and the anodes of each diode coupled to each other and to the first and second feedback resistors.